Climate fix could deplete polar ozone

Scientists seeking to cool Earth’s climate by injecting millions of tons of sulfuric acid droplets high in the atmosphere might trim rising temperatures but could also destroy much of the ozone in polar regions, a new study suggests.

Major volcanic eruptions spew large amounts of tiny particles, or aerosols, high into the atmosphere, where they scatter light back to space and significantly cool Earth for months to years (SN: 2/18/06, p. 110). Some researchers have proposed lofting tons of aerosols into the stratosphere to achieve the same result, but that process — often dubbed geoengineering — could have a number of detrimental side effects. Last year, for example, scientists noted that average precipitation worldwide dropped significantly in the 16 months immediately following the 1991 eruption of Mount Pinatubo (SN: 8/25/07, p. 125).

Now, count ozone destruction among the drawbacks of geoengineering. High-altitude ozone helps block damaging ultraviolet radiation from reaching Earth’s surface. Ozone-destroying chemical reactions occur most readily on the surfaces of high-altitude ice crystals and droplets of sulfuric acid spewed by volcanoes, says Simone Tilmes, an atmospheric scientist at the NationalCenter for Atmospheric Research in Boulder, Colo.

So, Tilmes and her colleagues estimated the ozone loss that would be triggered by two geoengineering scenarios, each designed to counteract the warming effect caused by doubling the pre-industrial atmospheric levels of carbon dioxide, as expected to occur late this century.

In one scenario, scientists inject about 2 million metric tons of sulfur-bearing aerosols into the stratosphere each year, each droplet approximately 0.46 micrometers in diameter. The other scenario lofts only 1.5 million metric tons of sulfur each year but in the form of smaller aerosols, which are more effective at scattering sunlight back into space.

Ozone destruction estimates are based on observations gathered during the last couple of decades, says Ross Salawitch, an atmospheric chemist at the University of Maryland, College Park. Results indicate that over the next few decades, ozone loss high above the Arctic after a particularly cold winter — one that produced large numbers of high-altitude ice crystals — could approach 75 percent, Tilmes, Salawitch and their colleagues report in an upcoming Science.

The effects of sulfate-aerosol geoengineering would be smaller later this century than today, primarily because atmospheric levels of ozone-destroying chemicals such as chlorofluorocarbons are now declining. Nevertheless, injecting sulfates into the atmosphere could delay the recovery of the ozone hole over Antarctica by 30 to 70 years.

Ozone loss due to geoengineering “is a real concern, but I don’t see it as a showstopper,” says Ken Caldeira, a climate modeler at the Carnegie Institution of Washington in Stanford, Calif. Even in the worst case cited by Tilmes and her colleagues, polar residents would experience levels of ultraviolet radiation no higher than those routinely seen in San Diego today, he contends. There are several ways to address detrimental side effects of geoengineering, he suggests. For example, scientists could design the aerosols to drop out of the atmosphere before they reach polar regions, where they wreak most of their havoc.

Other researchers aren’t so sanguine. The new research is “a valuable first step that shows both the limits and the strengths of such analyses,” says Michael J. Mills, an atmospheric scientist at the University of Colorado, Boulder. “Climate is a complex system, and before we do something like this, a lot more modeling needs to be done.”

“It’s always been clear that geoengineering would have some detrimental effect, but this paper quantifies it,” says Bill Chameides, an atmospheric chemist at DukeUniversity in Durham, N.C. Also, he notes, masking the planet-warming effects of carbon dioxide emissions rather than reducing them doesn’t do anything to reduce ocean acidification, another harmful side effect of burgeoning atmospheric concentrations of carbon dioxide (SN: 3/15/08, p. 170).

Among other uncertainties in geoengineering, it would be tough to fine-tune the lifetime, composition and size distribution of aerosols being injected into the atmosphere, says Adrian Tuck, formerly an atmospheric scientist at the Earth System Research Laboratory in Boulder, Colo. “Most of us share worries about geoengineering, which is seen as a cure-all to avoid having to bite the bullet about carbon emissions,” he adds.